1. Field of the Invention
The present invention relates to a battery pack, and more particularly, to a battery pack which can prevent easy detachment of a welded part between a cap plate and a case due to external impact, thereby enhancing hermeticity and reliability.
2. Description of the Related Art
Secondary batteries are rechargeable and can be repeatedly used. Thus, secondary batteries are more economical than disposable batteries.
Also, the secondary batteries can realize high capacity with small volume, so that they are widely used as a driving power source for handheld electronic devices such as cellular phones, notebook computers, camcorders and digital cameras.
In addition, owing to the high capacity of the secondary battery, the secondary batteries are widely used in high power products such as hybrid cars and electric tools as well as handheld electrical appliances.
Such secondary batteries include, for example, nickel-cadmium batteries, nickel-metal hydride batteries, nickel-zinc batteries, lithium ion secondary batteries and lithium polymer secondary batteries.
The lithium secondary batteries are widely used because they are compact and have high capacity, a high operating voltage and a high energy density per unit weight.
The lithium secondary batteries are formed by accommodating an electrode assembly and an electrolyte in a case and sealing the case.
The lithium secondary batteries may be classified into a can type and a pouch type depending on the shape of the case, and the can-type batteries may be classified into a cylindrical type and a rectangular type.
The rectangular-type lithium secondary battery is formed by accommodating an electrode assembly including a positive electrode plate, a negative electrode plate and a separator in a rectangular-type case having an opening at one side, and sealing the opening using a cap assembly.
Here, the cap assembly includes a cap plate disposed at an opening of the case, and an electrode terminal inserted into in the middle of the cap plate.
The case is sealed by welding an edge of the cap plate disposed at the opening using laser to join the cap plate with the case.
Since the rectangular-type secondary battery has the electrode terminal in the middle of the cap plate, the welded part between the cap plate and the case can be easily detached from each other when external impact caused, for example, by the battery being dropped, is concentrated in a part where the electrode terminal is disposed.
Moreover, since there is a hole in the cap plate to insert the electrode terminal into the cap plate, the hole part of the cap plate is more vulnerable to the external impact.
Recently, a rectangular-type secondary battery which has a larger width than a height is being developed. However, when the width is larger than the height, the resistance to damage as a result of external impact applied to the cap plate is relatively weak, so that the cap plate and the case can be more easily detached from each other.